Direct observation of valence tautomerism in sublimated molecular sub-monolayers

This study reports the first observation of a sub-monolayer molecular deposit prepared by sublimation on a conductive substrate of valence tautomerism (VT), an entropy-driven rearrangement of valence electrons between a metal ion and an organic ligand in redox-labile coordination compounds. To this end, a neutral cobalt(II) complex including two iminosemiquinonate redox-active ligands was thermally sublimated under UHV conditions onto highly oriented pyrolytic graphite (HOPG). Scanning tunnel microscopy and UV photoelectron spectroscopy, flanked by DFT calculations, were used to investigate the morphology and occurrence of tautomeric equilibria at the monolayer level. Temperature-dependent X-ray photoelectron spectroscopy (XPS) experiments provided evidence of the intactness of the molecular deposit and reversible VT behavior, which is characterized by an incipient, reversible transition at T > 300 K. Finally, the quantitative estimation of the phenomenon was achieved using soft X-ray absorption spectroscopy (XAS), which allowed the determination of the effect of surface deposition on the thermodynamics of the VT process. These findings establish the feasibility of preserving reversible valence tautomerism in surface-supported molecular layers and highlight the potential of VT compounds as active components in hybrid molecular spintronic architectures.